Potable water being delivered to a cistern in San Miguel de Allende (C) Daniel Friedman Effective drinking water disinfection
What water contaminants may be left after treating water with a disinfectant

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Limitations of disinfection's ability to produce safe, potable drinking water: this article describes the limitations of relying only on disinfection (chlorine or other disinfectants) to make drinking water safe and potable.

We explain that some biological or pathogenic drinking water contaminants are either resistant to standard disinfection approaches such as chlorination while other water contaminants such as hazardous chemicals or particulates are simply not addressed by disinfection.

Our page top photo shows potable water being delivered to a storage cistern in San Miguel de Allende, Guanajuato, Mexico.

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Is Water Disinfection Sometimes Ineffective?

There is growing evidence that drinking water disinfection alone can be ineffective in adequately removing drinking water contaminants, both because of procedural errors (discussed here) and possibly because of persistant contaminant sources or contaminants that are partly-encapsulated inside water piping or containers, requiring extra contact time or additional cleaning and disinfection methods to assure a stable sanitary drinking water supply.

In August 2013 The New York Times reported that in the United States drinking water chlorination was first installed in a municipal water supply in Jersey City New Jersey and in Chicago in 1908, leading today to more than 53,000 water supply utilities in that country providing "some of the safest drinking water in the world".

But the article continued to point out that there is growing concern for the effectiveness of chlorination in adequately treating a variety of pathogens. - The New York Times, 27 August 2013

This is not a new water quality concern. Payment (1999) explain that residual chlorine present in drinking water distribution systems was not effective in reducing organisms other than E. coli. Here is an excerpt from the article abstract

Except for Escherichia coli, microorganisms remained relatively unaffected in water from the distribution systems tested. When sewage was added to the water samples, indigenous thermotolerant coliforms were inactivated only when water was obtained from sites very close to the treatment plant and containing a high residual chlorine concentration.

Clostridium perfringens was barely inactivated, suggesting that the most resistant pathogens such as Giardia lamblia, Cryptosporidium parvum, and human enteric viruses would not be inactivated.

Our results suggest that the maintenance of a free residual concentration in a distribution system does not provide a significant inactivation of pathogens, could even mask events of contamination of the distribution, and thus would provide only a false sense of safety with little active protection of public health.

More recently, Richardson (2003) noted that in addition to the pathogen resistance problem we've just noted, disinfection byproducts in drinking water are also hazardous. We discuss DBPs in more detail at

Lazarova (1999) pointed out that there was considerable variation in the effectiveness of chlorination as a water disinfectant depending on the beginning water quality:

Chlorination/dechlorination and advanced disinfection processes (UV irradiation, ozonation, membrane filtration) have been reviewed in terms of their efficiency, regrowth potential, design parameters, experimental set-up, scale-up and industrial experiences. Existing results show the great influence of water quality, in particular of suspended matter concentration and organic content.

... The critical analysis of the literature data and experimental results highlights UV irradiation as an effective and competitive advanced disinfection process. Ozonation is a viable solution in case of higher requirements for water quality including virus and protozoa removal.

Ultrafiltration is a highly efficient process producing an excellent quality and totally disinfected effluent, particularly recommended for groundwater recharge and potable wastewater reuse. The choice between these advanced disinfection technologies depends on wastewater quality, existing standards, specific reuse applications and wastewater treatment work capacity. - Lazarova (1999)

LeChevallier (1999) explain that the turbidity of water affects its successful disinfection using chlorination:

To define interrelationships between elevated turbidities and the efficiency of chlorination in drinking water, experiments were performed to measure bacterial survival, chlorine demand, and interference with microbiological determinations. Experiments were conducted on the surface water supplies for communities which practice chlorination as the only treatment.

Therefore, the conclusions of this study apply only to such systems.

Results indicated that disinfection efficiency (log10 of the decrease in coliform numbers) was negatively correlated with turbidity and was influenced by season, chlorine demand of the samples, and the initial coliform level.

Total organic carbon was found to be associated with turbidity and was shown to interfere with maintenance of a free chlorine residual by creating a chlorine demand. Interference with coliform detection in turbid waters could be demonstrated by the recovery of typical coliforms from apparently negative filters.

The incidence of coliform masking in the membrane filter technique was found to increase as the turbidity of the chlorinated samples increased. the magnitude of coliform masking in the membrane filter technique increased from less than 1 coliform per 100 ml in water samples of less than 5 nephelometric turbidity units to greater than 1 coliform per 100 ml in water samples of greater than 5 nephelometric turbidity units. Statistical models were developed to predict the impact of turbidity on drinking water quality. T

he results justify maximum contaminant levels for turbidity in water entering a distribution system as stated in the National Primary Drinking Water Regulations of the Safe Drinking Water Act. - LeChevallier (1999)

Examples of Water Disinfection Limitations

Watch out: as we report throughout this article series, different disinfection methods vary in their effectiveness in combating different types of water contaminants. If you rely on a single disinfection method, for example chlorine disinfection, your water supply could still be contaminated by cryptosporidium, or if chemical contaminants are present, those, too, might remain.

Municipal water supplies are generally safe as their water treatment efficacy is monitored regularly as required by federal regulations. But private water supplies may be at risk.

Consulting with your local health department and local water testing laboratories can provide locally-accurate advice on what water tests are most important to perform in order to understand what water treatment may be necessary for your home or building.

Drinking water disinfection does not remove chemical contaminants

If the drinking water has been tested only for bacteria and subsequently treated only with disinfection, there is a risk that other contaminants may still be present in the water. In our experience this is partiularly the case if a private water well has become contaminated by ground water leaking into the well.

Groundwater or surface-runoff easily pick up surface contaminants such as road salt, pesticides, weed killers or other chemicals used above ground anywhere where these substances may be transported to a leak into a well.

Disinfectants that don't kill cysts

The Times article continued to note that following earlier disease outbreaks principally due to protozoan cysts of the parasite Cryptosporidium, the U.S. EPA added additional regulations requiring additional treatment for municipalities like New York city who were using un-filtered surface-reservoir water supplies.

New York City was reported to have instituted use of ultra voilet light to treat Cryptosporidium cysts that are otherwise resistant to just chlorination disinfection methods.


Persistent Bacterial Sources in Water & Limitations of Chlorine Disinfection of Drinking Water

We have observed many instances of effective well shocking that resulted in a "passed" water test only to find that bacterial contamination has reappeared in the well water days or weeks after the well shock procedure. In these cases most often we suspet that there was a persistent bacterial source that was simply temporarily dealt-with by the disinfectant. Persistent bacterial water contamination sources might be

The New York Times article cited above pointed out that testing for coliform bacteria, the most common water "potability test" does not include testing for even other microorganisms such as methylobacteria, sphingomonads, mycobacteria, and the Giardia or Cryptosporidium cysts reported above. Bacteria located within protozoan cysts can survive in that form for literally hundreds of years if not effectively treated.

Schoenen (2002) pointed out that despite the use of chlorine disinfection of drinking water, water-bourne disease outbreaks have continued around the world, particularly where there are fecal contaminants in drinking water!

Transmission of pathogens with drinking water is a widespread problem, which affects not only the countries with low hygienic standards but the industrialized countries as well. The pathogens are excreted by man or animals and are picked up orally. Chlorination of drinking water has been introduced to the water supply in the beginning of the 19th century in order to stop the spreading of pathogens especially typhoid fever by drinking water.

Despite the worldwide use of chlorine for disinfection of drinking water, water-mediated disease outbreaks occur again and again. Disinfection of drinking water with chlorine has undoubtedly contributed to the reduction of typhoid fever mortality. However, it must be clear that other factors play an important role in the mortality drop. Filtration of water is a long-known and very effective process for eliminating pathogens from the drinking water.

Pathogens in particles cannot be killed sufficiently by a chemical disinfectant. Even small fecal particles have to be eliminated reliably from the water by filtration. Disinfection of drinking water cannot replace filtration. The disinfection should be used to minimize the residual risk due to the presence of pathogens in the water but cannot be used for bringing fecally contaminated water into a hygienically sound condition. - Schoenen (2002)

Previously Xu (1996) pointed out that pathogens entrapped in biofilms can limit the rate (and thus effectiveness) of chlorine disinfection:

An artificial biofilm system consisting of Pseudomonas aeruginosa entrapped in alginate and agarose beads was used to demonstrate transport limitation of the rate of disinfection of entrapped bacteria by chlorine. Alginate gel beads with or without entrapped bacteria consumed chlorine. ... Chlorine fully penetrated cell-free agarose beads rapidly; the concentration of chlorine at the bead center reached 50% of the bulk concentration within approximately 10 min after immersion in chlorine solution.

When alginate and bacteria were incorporated into an agarose bead, pronounced chlorine concentration gradients persisted within the gel bead. Chlorine did gradually penetrate the bead, but at a greatly retarded rate; the time to reach 50% of the bulk concentration at the bead center was approximately 46 h. ...

Spatially nonuniform killing of bacteria within the beads was demonstrated by measuring the transient release of viable cells during dissolution of the beads. Bacteria were killed preferentially near the bead surface. Experimental results were consistent with transport limitation of the penetration of chlorine into the artificial biofilm arising from a reaction–diffusion interaction. The methods reported here provide tools for diagnosing the mechanism of biofilm resistance to reactive antimicrobial agents in such applications as the treatment of drinking and cooling waters. - Xu (1996)

Conversely, chlorine disinfection of drinking water may be more effective against some troublesome biological contaminants than previously thought: Shin (2000) discuss the resistance of a particular contaminant, norovirus, to chlorine disinfection of drinking water with a surprising result.

In an effort to validate previous research suggesting remarkable resistance of norovirus to free chlorine disinfection, we characterized the disinfection response of purified and dispersed Norwalk virus (NV) by bench-scale free chlorine disinfection using RT-PCR for virus assays. The inactivation of NV by two doses of free chlorine (1 and 5 mg/L) at pH 6 and 5 °C based on two RT-PCR assays was similar to that of coliphage MS2, but much faster than that of poliovirus 1.

Despite the underestimation of virus inactivation by RT-PCR assays, the predicted CT values for NV based on RT-PCR assays are lower than the ones for most other important waterborne viruses and the CT guidelines for chlorine disinfection of viruses under the Surface Water Treatment Rule by the United States Environmental Protection Agency.

Overall, the results of this study indicate that NV is not highly resistant to free chlorine disinfection as suggested by previous research and it is likely that NV contamination of drinking water can be controlled by adequate free chlorine disinfection practices with provision of proper pre-treatment processes before chlorination. - Shin (2000)

Water disinfectant effectiveness limited by water pH

We explain in this separate but important article that WELL DISINFECTANT pH ADJUSTMENT may also be necessary for effective water disinfection.


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